Unify structure for ext/random's engine tests (#9321)

* Remove user_xoshiro128plusplus.phpt

This test does not exercise any of the code paths of the random extension and
thus is

Unify structure for ext/random's engine tests (#9321)

* Remove user_xoshiro128plusplus.phpt

This test does not exercise any of the code paths of the random extension and
thus is no value-add.

* Unify structure for ext/random's engine tests

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Implement Random Extension

https://wiki.php.net/rfc/rng_extension
https://wiki.php.net/rfc/random_extension_improvement